Population Biology
Objectives:
•Explain how populations grow
•Identify Factors that inhibit the
growth of a populations.
•Summarize forces behind and
issues human population growth.
Population Growth
An increase in the size of a
population over time.
Researchers Study Populations
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Geographic range
Density and distribution
Growth rate
Age Structure
METHODS USED TO INVESTIGATE
POPULATION GROWTH IN ORGANISMS:
•Lab-Place microorganisms, such as bacteria or yeast cells, into tube or
bottle or nutrient solution and observe how rapidly the population
grows.
• Field-Introduce a plant or animal species into a new environment that
contains abundant resources and then observe the population growth of
that species.
Geographic Range
• Area inhabited by a population
• Can be huge, depending on species
– Bacteria on rotting pumpkin (smaller than cubic
meter) to cod in Atlantic Ocean (Canada to North
Carolina
the number of individuals per unit of
PopulationDensity area
Distribution
How individuals in a population are spaced out
across the range of the population.
• Randomly
• Uniformly
• Clumped
Difference between birthrate and death rate.
GROWTH RATE
Calculating Growth Rate—use
“doubling time”
•Time needed for any population to
double its size.
•Doubling Time (years) = 70/ Annual
Percent Growth Rate
Age Structure: number of males and females of
each age a population contains-can help predict
growing rapidly, slowly or not at all.
• Differences in environmental conditions and past
history may cause populations to differ in their age
distributions.
• The future growth of a population depends on its
current age distribution.
Factors that can affect
population size
• Birthrate
• Death rate
• Rate at which
individuals enter or
leave the population
Birthrate and Death rate
• Birthrate higher than death rate-populations
grow
• Death rate higher than birthrate-populations
likely to shrink
• Birthrate = death rate – population stays same
size
United States Trends
• Death rates have greater effect on
total populations
• Life Expectancy increases almost
every year
Birthrates
• Fertility rate decreasing b/c people
waiting longer to have kids
• Fewer children
Immigration: Movement of individuals into a population
Emigration: Movement of people between countries
No effect on world population, but does affect national population.
Migration: Movement in or out of a population
MOBILITY
•Not Linear (straight line)
•J-shaped –Initially growth is slow, then rapid growth due to total number
of reproducing organisms (exponential growth)
•As population gets larger, it grows faster
•Unlimited resources
EXPONENTIAL GROWTH
Life-history Patterns
Rapid Life-history patterns
•Unpredictable and change rapidly
•Typically small body size, mature rapidly, reproduce early and have
short life span.
•Populations increase rapidly and decline rapidly in unfavorable
conditions.
•e.g. mosquitoes, bacteria
Long Life-history patterns
•Reproduce and mature slowly.
•Maintain population size near carrying capacities.
•e.g. elephants, humans, bears, whales, cacti, bristlecone pine
Organisms in new environments
• Move in new place, populations grows
exponentially over time
• E.g. exotic species; European gypsy moth
Populations DO NOT grow
indefinitely! (Level-off)
Limiting factors
•Food
•Space
Logistical Growth
When a population’s growth slows
and then stops, following a period of
exponential growth. (s curve)
Logistical Growth (s curve)
• Phase 1-Exponential Growth
– Population grows exponentially (J shaped curve)
– Resources are unlimited
– Growth and reproduction rapid
– Population size and rate of growth are increasing
more and more rapidly
Logistical Growth
• Phase 2-Growth Slows Down
– Population growth slows down
– Does not mean population size decreases
– Rate of growth slows, population increasing more
slowly
Logistical Growth
• Phase 3-Growth stops
– Some point rate of growth drops to zero
– Levels off
– Under same conditions-population will remain
near or at this size indefinately.
Carrying Capacity
Number of organisms that an environment
can support.
Carrying Capacity
Reached
• Birth= Death
Population
Overshoots
Carrying Capacity
• Deaths exceed births until
carrying capacity reached
• Population tends to
decrease
Population Falls
Below Carrying
Capacity
• Births Exceed Deaths until
carrying capacity
• Population tends to
increase
Reproductive pattern is the most important factor that
determines population growth
Limiting Factors
Factor that controls growth of a
population.
Types of limiting factors
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Competition
Predation
Parasitism and disease
Unusual weather
Natural disaster
Density-Dependent Limiting
Factors
Operate strongly only when population density
reaches certain levels.
Density Independent Factors
Factors that affect all populations in
similar ways, regardless of population
size and density.
Limits to Population Growth
Density Dependent
• Disease, competition,
parasites, predation,
herbivory, parasitism, stress
and overcrowding, and food
• Increasing effect as population
increases
• e.g. more dense more quickly
disease spreads.
Density Independent
• Affect all populations,
regardless of density
• Abiotic factors-temperature,
storms, floods, drought,
habitat disruptions, pollution.
• Human activity
• Controlling Introduced
Species-herbicides and
mechanic removal
• e.g. Cold temps kill
mosquitoes
Acting separately or together,
limiting factors determine the
carrying capacity of an
environment for a species
Organisms Interactions Limit
Population Size
• Predation-Prey-cycles of increases and
decreases over time
• Competition for Resources—Food, water and
territory (Demand exceeds supply, populations
decrease)—Density Dependent
• Effects of Crowding and Stress (aggression,
decrease in parental care, decreased fertility
and decreased resistance to disease)
Predator-Prey
Relationships
Human Population Growth
Historical Overview
• Human populations tend to increase
• Rate of that increase has changed over time:
• Then, Now and Future
– Early human existence
– Exponential Human Population Growth
– Predictions of Malthus
– World Population Growth Slows
Human Population Growth
– For most of human existence growth was slow
• Limiting factors kept death rates high; Harsh life; food
hard to fine; predation and disease; many children did
not survive to adulthood.
Human Population Growth
– Exponential human population growth
• Civilization advanced, life easier; human population
growth began to grow more rapidly
• 1800 – industrial revolution
• Improvement in nutrition, sanitation, medicine, and
healthcare reduced death rates.
Historical Overview Continued
• Predictions of Malthus
– English Economist Thomas Malthus suggested that
only war, famine and disease could limit human
popultion growth.
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Regulated by competition (war)
Limited resources (famine)
Parasitism (disease)
Other density-dependent factors
World Population Growth Slows
• Exponential growth continued until the
second half of the 20th century.
• Human Growth rate reached its peak b/w
1962-1963, and then began to drop.
• Size of global human population growing; rate
slowing down.
– 1804 (1 billion) to 1927 (2 billion)- 123 years to
double
– 33 years to double to 3 billion
– Now takes longer for global human population to
grow by 1 billion
Patterns of Human Population
Growth
DEMOGRAPHIC TRENDS
Demography- Study of human population growth characteristics.
Demographers study growth rate, birthrates, death rates, age structure,
and geographic distribution. Help predict why some countries have high
growth rates and others grow more slowly.
Humans reduce environmental effects by eliminating competing organisms, increasing
food production, and controlling disease organisms.
Demographic Transition
• Dramatic change from high birthrates and
death rates to low birthrates and death rates.
• US, Japan and European completed
• 3 Stages:
– Stage 1- Birthrate and death rate = high
– Stage 2 – Birthrates high, death rates low
– Stage 3- Birthrates and death rates = low
Age Structure and Population
Growth
• Compares age structures in
different countries
• Us-nearly equal numbers of
people in each age grouppredicts slow, steady growth
• Guatemala – more young
children and teens –predicts
rapid population growth
Future Population Growth
• To predict future
– Age structure for a country
– Effects of disease on death rates
• Current models- 9 billion by 2050
• Data suggests-global human population will
grow more slowly than it has been growting